The first gene therapy protocol in the United States was begun in 1990 for the treatment of severe combined immunodeficiency
(1). Overall, gene transfer has been used most commonly to treat cancer, followed by monogenetic, infectious, cardiovascular, neurological, and ocular diseases (
Table 69-1). Recent success has been reported in treating patients with severe combined immunodeficiency
(2), a fatal demyelinating disease of the central nervous system (
3,
4), and an inherited retinal disease causing congenital blindness
(5). As of 2009, 1,579 gene transfer protocols have been initiated worldwide (
6). Of these, approximately 60% are in Phase I trials, 35% in Phase I/II or II trials, and the remainder in Phase III or IV trials. Only two gene therapy products have been marketed, and these are in China.
Clinical gene transfer trials reflect our developing understanding of the genetic basis of many diseases and rapid advances in molecular biology including the ability to produce vectors capable of transferring genetic material into somatic cells. However, the need for careful assessment of the potential benefits and risks of all gene therapy trials has been highlighted by the unexpected death of a young patient that was directly attributable to the gene transfer trial
(7) and the development of leukemia in several patients who underwent retrovirus-mediated gene transfer to correct X-linked severe combined immunodeficiency syndrome (
8,
9).
Several live pathogenic viruses have been modified to transfer genes of interest. The ability of these vectors to infect patients (and potentially other unintended persons) raises considerations for infection control. This chapter provides an overview of gene transfer technology and regulatory requirements for research in the United States and discusses the infection control aspects of clinical trials using gene transfer.
Recommendations for infection control of gene therapy/transfer have been discussed in an editorial (
10), consensus conference (
11), and a review article (
12).
BACKGROUND
Gene transfer is a term that can be applied to any clinical therapeutic procedure in which genes are intentionally introduced into human somatic cells
(13). Prior to considering gene transfer, several requirements must be fulfilled. First, the gene(s) in question must be identified, and the nature of the defect characterized. Genetic diseases can be defined by the aberrant, specific gene expression that differs from the disease-free state. This variance may be due to a gene product that is absent or deficient (e.g., the cystic fibrosis transmembrane regulator (CFTR) protein)
(14,
15), one that is abnormally present (e.g., Epstein-Barr virus nuclear antigen-1 in Hodgkin’s disease)
(16), or abnormal regulation or expression of normal cellular products (i.e., downregulation of human leukocyte antigens by adenovirus). Second, it is important to understand which tissues express the defect and how accessible they are to manipulation. For example, while hemophilia B is caused by inadequate production of factor IX by the liver, factor IX does not require precise metabolic regulation, and even small amounts of production of factor IX by any cell line can prevent disease manifestations. Thus, hemophilia B is potentially amenable to
ex vivo manipulation of hematopoietic cells or fibroblasts
(17). The key technologies that have facilitated the utilization of gene transfer include new methods by which cellular genes can be isolated (cloned), manipulated (engineered), and transferred into human cells. To obtain a therapeutic effect, there are basically three options for somatic gene therapy: (a) replacement of defective or missing genes for the treatment of inherited diseases, (b) augmentation of normal gene function or introduction of additional genetic information that interferes with proliferative diseases, and (c) blocking disease triggering or supporting genes like oncogenes on the deoxyribonucleic acid (DNA) or ribonucleic acid (RNA) level (
Table 69-2). In brief, these three options could be thought of as gene replacement, gene addition, or gene correction
(18).Human gene transfer is currently limited to manipulations affecting somatic, differentiated cells. Germline gene transfer, where reproductive cells are treated for the correction of a genetic disease being transferred to the patient’s descendants, is not likely to become acceptable as a feasible strategy in the near future. Due to the potential risks and unpredictable results, germline gene transfer has never been authorized in humans.
There are two main approaches to gene transfer:
in vivo gene transfer, in which genes are delivered directly to target cells in the body, and
ex vivo gene transfer, in which
target cells are genetically manipulated outside the body and then reimplanted
(13). To carry out gene transfer, the exogenous gene(s) is transferred in an expression cassette, including the promoter, which regulates expression of the new gene, often in the form of a complement DNA (cDNA), and stops signals to terminate translation
(19). The exogenous or therapeutic gene can be isolated from the genome of a human, another animal, a plant, a bacterium, or a virus and may code for any type of protein
(13). Depending on the choice of the regulatory element, which controls the expression of the therapeutic gene, gene expression can be high or low level, specific to certain cell types, or even continuously variable, and can respond to local environmental factors such as the partial pressure oxygen or the concentration of a drug
(13).The expression cassette is transferred to target cells using a vector. The most commonly used vector systems include retroviruses, lentiviruses, adenovirus, adenoassociated virus, poxviruses such as vaccinia, and herpes simplex virus (
Table 69-3). Each delivers the expression cassette via distinct mechanisms and each has unique advantages and disadvantages (
Table 69-4). Although viral vectors have been most commonly used, nonviral vector systems are of increasing scientific interest. Nonviral vector systems include plasmid-liposome complexes, newer kinds of vectors that sheath DNA in nonlipid coats, and naked DNA
(20,
21 and 22).To date, the many obstacles to successful gene therapy/transfer have not been overcome. The ideal gene delivery vehicle would efficiently and specifically transfer the gene to target cells and subsequently obtain high, regulatable, and durable levels of gene expression
(19). In addition, an ideal vector should not evoke an immune response (unless designed to do so), should be nontoxic to the recipient and easily purified in high concentration, and there should be no risk of recombination or replication (unless desired). Current obstacles to successful gene therapy include low efficiency of gene transfer to the target cell, inadequate regulation of the therapeutic gene in the transduced cell, and maintaining long-term, stable gene expression at an appropriate level.
